There are several things that can prevent a computer from successfully completing startup. The first step in figuring out what may be causing a problem is to determine whether the problem is occurring before the operating system takes control. If you do not see the boot loader screen on an x86-based computer, the problem may be due to a hardware failure, the Master Boot Record, Partition Tables, or the Partition Boot Sector, which might be damaged.
There are several ways such damage can happen, including viruses. On x86-based computers, viruses use BIOS interrupt (INT) 13 calls to install themselves, so they are operating-system independent. Windows NT traps BIOS INT 13 calls while it is running, but cannot protect itself when the computer is started from an MS-DOS bootable floppy disk or is dual-booted by using MS-DOS. The Knowledge Base contains several articles about protecting your computer from viruses and recovering from problems with viruses.
If the problem occurs after selecting Windows NT from the boot loader screen, files that are needed by the operating system might be missing or corrupt.
If you have installed new hardware or new drivers, they could be causing the problem.